The problem addressed by the present invention can arise in various contexts. One of these is a new generation of Integrated Services Digital Network (ISDN) telephone sets currently being developed. These sets employ softkeys, which, as is well known, are buttons that are each associated with a display label that displays an alphanumeric character string indicative of the meaning of the associated button at any particular time. Such an arrangement permits the meaning of the buttons to be changed to best suit the particular context in which the button is being used at any particular time. For such telephone sets, the meaning of any button and the string that should be displayed on its corresponding display label are specified by a telephone switch in a telephone central office to which the set is connected.
Not all ISDN telephone sets, however, will necessarily have the same length display label for the same associated button. For example, it is expected that there will ultimately be a great number of manufacturers of ISDN telephone sets, each purveying a variety of ISDN telephone sets and featuring a range of display label lengths. The problem, then, is how to convey, from the central office to the station set, a message indicative of the meaning of a button so that a string that will fit within the particular length of the button's associated display label, and is best indicative of the intended meaning, can be displayed.
One prior art method to specify such strings to an ISDN telephone set is to fix within the ISDN telephone set itself the best version of each string to be displayed on its particular display label length. The central office sends messages indicating which meaning should be given to each button at any time and the station set retrieves and displays the corresponding stored string. Another approach is to have each ISDN telephone set identify itself by a particular classmark that indicates its capabilities, including its display label length, to the central office. Once it has the classmark, the central office knows enough information about the display label lengths of the set so that it can send strings of appropriate length for display.
Unfortunately such prior art methods require many characteristics of the ISDN system to be static and/or defined prior to developing the system and, therefore, they are not satisfactory for use with ISDN. This is because the current expectation is that ISDN will change and evolve over time, thereby requiring new meanings and/or station set configurations while obsoleting old ones. Such evolution is antithetical to a system having many static characteristics that are defined prior to implementation. Also, there are expected to be variations among the ISDN services as they are provided by different service providers. Fixing the strings in the station set would not easily allow for such variations
As a result, it is desirable that a message containing an indication of the string to display should be conveyed to the ISDN telephone set by the central office while making the station set responsible for insuring that a meaningful string is displayed within the display label without prestoring all possible strings. To do so, the prior art requires that a maximum length string, corresponding in length to a maximum label length, be developed for each meaning and conveyed, at the proper time, from the central office to an ISDN telephone set. Any station set that receives the string and cannot display it in its entirety abridges it to fit within its particular label length. One simple abridgment method is truncation, wherein all the characters that fit on the label, starting with the leftmost one, are displayed and any characters that are further to the right than the last displayable character are simply dropped. This is less than ideal because, generally, it is not possible to arrange a string of characters in such a way that it will read well regardless of how much of its rightmost portion is truncated. For example, the term "Auto Callback" using truncation for a label length of 8 characters would be "Auto Cal" (since spaces count as characters on a display). However, the string "Auto Cal", the eight-character truncation of "Auto Callback", provides much less intuitive information to a user than the term "Callback", which also contains 8 characters, but cannot be achieved with truncation. Similarly "Aut", the three character truncation of "Auto Callback", is totally uninformative, as compared with "ACB", which would also fit on a 3 character display.
The dropping of vowels, another simple alternative abridgment technique, may achieve better results than truncation for certain strings. However, neither the dropping of vowels nor truncation works well in all cases.